1. Field of the Invention
This invention relates to a process for electrochemical reduction of CO.sub.2 to CH.sub.4 and C.sub.2 H.sub.4 providing both high current densities and high Faradaic efficiencies. The process is carried out in an electrochemical cell wherein copper is electrodeposited in situ on the cathode surface during at least initial cell operation. Faradaic efficiencies of 73 percent for CH.sub.4 and 25 percent for C.sub.2 H.sub.4 at a current density of 8.3 mA/cm.sup.2 have been obtained.
2. Description of the Prior Art
Considerable effort has been expended towards promoting the electrochemical reduction of CO.sub.2 to useful hydrocarbons at both high Faradaic efficiencies and high current densities. While a number of chemical catalysts have been identified for CO.sub.2 reduction to methane and higher hydrocarbons in the gas phase, relatively few catalysts have been identified for electrochemical reduction of CO.sub.2 to hydrocarbons in an aqueous electrolyte.
Indirect reduction of CO.sub.2 on a mercury electrode in an aqueous electrolyte, pH 7, containing TiCl.sub.3, Na.sub.2 MoO.sub.4 and pyrocatechol where the total Faradaic efficiency for cathodic hydrocarbon generation was about 0.2 percent at 7 mA/cm.sup.2, with methane being the major hydrocarbon component, is taught by Petrova, G. N. and O. N. Efimova, Elektrokhimiya, 19(7), 978 (1983). CO.sub.2 has been shown to be reducible to CH.sub.4, CO, and methanol at ruthenium cathodes in CO.sub.2 saturated aqueous Na.sub.2 SO.sub.4 electrolyte with Faradaic efficiencies for CH.sub.4 production up to 42 percent at current densities up 0.11 mA/cm.sup.2 by Frese, Jr., K. W. and S. Leach, J. Electrochem. Soc., 132, 259 (1985).
Copper, 99.99 percent pure, was used as a cathode with 0.5M KHCO.sub.3 electrolyte for the electrochemical reduction of CO.sub.2 at ambient temperature and current density of 5.0 mA/cm.sup.2 for 30 to 60 minutes with Faradaic efficiencies for CH.sub.4 of 37 to 40 percent, Hori, Y, K. Kikuchi and S. Suzuki, Chem. Lett., 1695 (1985). In later work, high purity copper cathodes, 99.999 percent, were used for the electrochemical reduction of CO.sub.2 in 0.5M KHCO.sub.3 electrolyte in a cell operated at a current of 5 mA/cm.sup.2 for 30 minutes at temperatures of 0.degree. C. and 40.degree. C., shows Faradaic efficiency for production of CH.sub.4 drops from 60 percent at 0.degree. to 5 percent at 4020 ; C.sub.2 H.sub.4 increases from 3 percent at 0.degree. to 18 percent at 40.degree.; while hydrogen production increases from 20 percent at 0.degree. to 45 percent at 40.degree.. It is stated that 99.99 percent pure copper cut the Faradaic efficiencies to about one-third of those obtained with 99.999 percent pure copper. Hori, Y, K. Kikuchi, A. Murata and S. Suzuki, Chem. Lett., 897 (1986). Later work of electrochemical reduction of CO.sub.2 at a 99.999 percent pure copper cathode in aqueous electrolytes of KCl, KClO.sub.4, and K.sub.2 SO.sub.4 at 19.degree. C. and current density of 5 mA/cm.sup.-2 showed Faradaic yields of C.sub.2 H.sub.4 of as high as in the order of 48 percent, CH.sub.4 12 percent and EtOH 21 percent. Hori, Y, A. Murata, Takahashi and S. Suzuki, J. Chem. Soc., Chem. Commun, 17, 1988.
Electroreduction of CO at a 99.999 percent pure copper cathode in an aqueous catholyte of KHCO.sub.3 at ambient temperature for 30 minutes showed hydrogen to be the predominant product, and at 1.0 mA/cm.sup.2, C.sub.2 H.sub.4 Faradaic production was 22 percent, CH.sub.4 1 percent; 2.5 mA/cm.sup.2 C.sub.2 H.sub.4 Faradaic production was 21 percent, CH.sub.4 16 percent and at 5.0 mA/cm.sup.2 C.sub.2 H.sub.4 Faradaic production was 16 percent, CH.sub.4 6 percent. Hori, Y, A. Murata, R. Takahashi and S. Suzuki, J. Am. Chem. Soc., 109, 5022 (1987). Similar work by the same authors showed electroreduction of CO at a 99.999 percent pure copper cathode in an aqueous 0.1M KHCO.sub.3 pH 9.6 catholyte at 19.degree. C. at 2.5 mA/cm.sup.2 resulted in Faradaic production C.sub.2 H.sub.4 of 21.2 percent; CH.sub.4 of 16.3 percent; EtOH of 10.9 percent; and 45.5 percent H.sub.2. Hori, Y, A. Murata, R. Takahashi and S. Suzuki, Chem. Lett., 1665 (1987).
In the reduction of CO.sub.2 to CH.sub.4 using 99.9 percent pure cold rolled B 370 copper cathodes with a CO.sub.2 saturated 0.5M KHCO.sub.3 electrolyte, Faradaic efficiencies of 33 percent were achieved for CH.sub.4 at current densities up to 38 mA/cm.sup.2 with no C.sub.2 H.sub.4 being detected. Cook, R. L., R. C. McDuff and A. F. Sammells, J. Electrochem. Soc., 134, 1873 (1987).
Electrochemical reduction of CO.sub.2 to CH.sub.4 and C.sub.2 H.sub.4 was shown to occur at copper/Nafion electrodes (solid polymer electrolyte structures) at Faradaic efficiencies of about 9 percent for each CH.sub.4 and C.sub.2 H.sub.4 at E=-200V vs. SCE using 2 mM H.sub.2 SO.sub.4 counter solution at a temperature of 22.degree. C. Dewulf, D. W., A. J. Bard, Cat. Lett. 1, 73, (1988).
The CO.sub.2 reduction has previously been indicated to be highly dependent upon platinum electrode surface morphology in the production of HCOOH. Czerwinski, A., J. Sobkowski and R. Marassi, Anal. Lett., 18, 1717 (1985). Simultaneous in situ deposition of nickel as an electrocatalyst in the electrochemical hydrogenation of organic molecules has proven effective for obtaining high activity catalytic sites. Lain, M. J. and D. Pletcher, Electrochim. Acta., 32, 99 (1987).